"Row" refers to the row of cooling fins in the core of the radiator. Number of rows has to be taken into consideration with the thickness of the core. For example, most radiators for the R51 are either a single row 2-1/4" core or a 2-1/8" double row core; either will work just fine.
"Drop in" means that it will (or "should") fit just like the factory part.
Replacing the radiator on an R51 Pathfinder is much harder than the earlier model (WD21, R50) Pathfinders. It's about a 3-hour job, depending on what tools you have, working environment, your experience, etc. The electric fan needs to be unplugged and disconnected from the shroud. The radiator shroud, electric fan and manual fan need to be removed, and coolant drained, of course. Radiator and trans cooler hoses need to be removed, which means the skid plate needs to be removed. Reservoir hose needs to be removed. The A/C condenser shares the lower bracket with the radiator and will need to be supported (tied up) when removing the radiator. Remove the top brackets, tilt the radiator forward, pull the condenser out of the lower bracket and support, pull the radiator up and out, maneuvering the side brackets attached to the radiator through the A/C liquid lines, which will remain attached to the condenser. Once the radiator is out, you can swap over the side brackets and, if you purchase it, install the new foam seals (you likely won't be able to get the old ones to stick onto the new radiator, if they are any good). Reverse to install. Fill the radiator with coolant and install the radiator cap. Fill the coolant reservoir to the "max" fill line. Install the reservoir cap, start the engine, turn all heaters to "maximum" heat setting, park on an incline with the nose up or jack up the front and let the air purge from the system.